Process for producing magnetic bodies



Patented Apr. 18, .1933

UNITED STATES PATENT. OFFICE LOW'ELL M. HOLLINGSWORTH, OF EAST ORANGE,NEW JERSEY, ASSIGNOB TO BELL TELEPHONE LABORATORIES, INCORPORATED, OFNEW YORK, N. Y., A. CORPORATION or nnw YORK PROCESS FOR PRODUCINGMAGNETIC BODIES Io Drawing. Application filed April 30,

This invention relates to magnetic bodies and particularly to processesfor producing magnetic bodies of the type employing magnetic material infinely divided form, for example, magnetic dust cores for loading coilsand the like.

An Object of the invention is to improve the magnetic and mechanicalcharacteristics of magnetic bodies of the type employing magneticmaterial in finely divided form.

Magnetic cores made from finely divided magnetic material, so-calledmagnetic dust cores, are being extensively used for theloading coilswhich are inserted in telephone lines to improve their transmissioncharacteristics. Such cores are usually constructed by subjecting thefinely divided particles of the magnetic material and a non-magneticmaterial between the magnetic particles to extremely high pressures inthe neighborhood of 200,000 pounds per square inch. The high pressuresare utilized to form the component materials into a substantiallyhomogeneous mass in the desired core form, and to give sufiicientmechanical strength to the completed core. The non-magnetic material isof such nature as to serve as a binder and also as insulation betweenthe magnetic particles so as to reduce eddy current loss in the core.

In the case especially when the magnetic material in the core made inthe above described manner is a magnetic alloy including nickel andiron, it has been found desirable to subject the core to a subsequentannealing heat treatment at a high temperature to remove the strainsintroduced in the magnetic material by the pressing operation, to removevolatile constituents of the insulating material tending to impair itsmagnetic stability, and to fully develop the magnetic characteristics ofthe alloy. In general, the magnetic properties of the magnetic materialimprove correspondingly as the temperature of this annealing heattreatment increases. Most insulating materials start to deteriorate froman insulating standpoint when subjected to heat treatment, before theoptimum value of annealing temperature for the magnetic material isreached, and the upper limit of the annealing temperature is thereforelimited by 1932. Serial No. 608,559.

the characteristics of the insulating material.

Heretofore, it has also been found desirable in order to obtain the bestmagnetic properties in magnetic bodies of the above described type tosubject the magnetic particles before they have been compressed intotheir final form, to a preliminary annealing heat treatment at a hightemperature, for example, in the manner disclosed and claimed in the U.S. patent to Karcher 1,669,665, issued May 15, 1928. Heretofore, it hasbeen the practice to subject the finely divided magnetic materials tothis preliminary annealing heat treatment before the insulation has beenapplied to their surfaces.

In accordance with the present invention it has been found that themagnetic and mechanical characteristics of magnetic bodies of the abovedescribed type may be improved, and the process of making them maybesimplified, if the preliminary annealing heat treatment of theindividual magnetic particles is carried out after the insulation hasbeen applied to their surfaces, so that the insulating material issubjected also to heat treatment at a high temperature prior to thefinal annealing heat treatment of the compressed magnetic body.

The invention efl'ectively increases for a given insulating materialused, the permissible temperature of the final annealing treatment inthe process of preparing magnetic bodies of the above described type,resulting in reduced magnetic losses for unit permeability.

The exact nature and advantages of the invention will be betterunderstood from the followingdetailed description t1. ereof.

In practicing the present invention, the magnetic material for themagnetic body or core is preferably prepared from a brittle alloycontaining nickel and iron, with or without the addition of otherconstituents, and commonly known as permalloy. The alloy may be preparedin the manner. described'in U. S. Patent 1,669,649, issued May 15, 1928,to C. P. Heath and H. M. E. Heinicke, wherein the magnetic metalsincluding nickel and iron in proper proportions, depending upon thedesired percentage composition of the alloy, are melted together in anoxidizing atmosphere, the resulting alloyl being poured into a mold.

T e alloy ingots so obtained, while still hot, are successively passedthrough progressively reducing rolls to decrease their crosssection tothe-required size, the final roll being preferably effected atapproximately the temperature at which the alloy ceases to be malleable,after which the rolled material is quenched in water at a temperaturebelow that at which it loses its malleability. This mechanical treatmentis for the purpose of producin a fine crystalline structure in thealloy, which is desirable since the disintegration of the material takesplace at the crystal boundaries, and consequently, the smaller the sizeof the .crystals in the alloy material, the finer the magnetic dustwhich can be produced therefrom. The resultant ingots are broken intoshort ieces and the pieces reduced to finely divided form in anysuitable manner, for example, by crushing in a rock crusher, hammermill, or other suitable aparatus and pulverizing the crushed material yrolling in an attrition or ball mill for several hours. After rolling,the resulting dust issieved through a -mesh sieve and that portion whichwill not pass through the sieve is returned to the ball mill for furtherrolling, this process being repeated until a sufiicientquantity of thefine dust is obtained.

Up to this point, the process of making the magnetic bodies is the sameas commonly practiced in the prior art. In the processes of the priorart, the. magnetic dust particles thus obtained would be then submittedto the preliminary annealing heat/treatment after which the insulatingcoatin would be applied to their surfaces. In t e process of the presentinvention, however, the procedure is reversed, that is, the insulatingcoatings are first applied to the magnetic dust particles and then theinsulated magnetic particles are subjected to the preliminary heattreatment at a suitable high temperature.

Any suitable insulating material and process for appl ing it may be usedfor insulating the indlvi ual magnetic dust particles in the process ofthe invention. Particularly good results have been obtained with theprocess of the invention using insulating materials and rocesses ofinsulating substantially as isclosed in Lathrop Patent 1,810,010, issuedAugust 11, 1931, the copendlng applications of H. Lathrop, Serial No.497,261, filed November 21, 1930 and Serial No. 548,404, filed July 2,1931, or those of C. C. Neighbors, Serial No. 505,956, filed December31, 1930 and 580,503, filed December 11, 1931.

The insulated magnetic dust particles are then annealed in a coveredcontainer at a temperature preferably. of approximately 850C. for aboutone hour or more.

It was found that when any of the insulating materials and processes ofinsulating disclosed in the above mentioned patents and applicationswere em loyed, the process of the invention enable ring annealingtemperature to be increased appreciably over the maximum core annealingtemperature allowable when the pr of the prior art were used. Core rinannealing temperatures as high as 650 did not cause appreciabledeterioration of the insulating materials in the com leted cores.

A pluralit of the rings so ormed are then stacked coaxially to form amagnetic core on which the usual loading coil toroidal windings arewound. The number of rings used in a given core will depend upon theexisting electrical characteristics ofthe telephone circuits with whichthe loading coils are to be associated.

A number of loading coil cores were made by the process of the inventionsimilar to that described above in which the preliminary annealin heattreatment of the magnetic dust partic es was carried out after saidparticles had been insulated, for comparison with other cores of similarconstruction made by a similar process but in which the preliminaryannealing heat treatment of the magnetic dust particles was carried outprior to their insulation in the usual manner. Tests on the I two lotsof cores showed that the former for a iven magnetic material and a giveninsu ating material had much lower hysteresis loss for unit permeabilitythan the latter, while the eddy current loss remained about the same. Inmost cases, it was found that with the process of the invention the hsteresis losses were reduced to approximate third those of the similarcores made y the processes of the prior art.

In addition to the above mentioned advantage of the process of theinvention, it was found that it'resulted in roducing magnetic coreshaving better mec anical characteristics than cores produced by the r ofthe prior art, robably due to't e fact that substantially al volatilematerial in the msulating materials is driven off in the pre- 3 liminaryheat treatment of the insulated magnetic dust particles. Still anotheradvantage of the process of the invention is that because the magneticparticles are insulated before the preliminary heat treatment, sinter- 1the permissible core one- 1 unit permeability ing of the magneticparticles in the annealing heat treatment is prevented and consequentlyless handling and mechanical working of the magnetic dust is required.

The obtaining of lower hysteresis loss for by the use of the process ofthe invention is of economic importance since it will permit bettertransmission quality to be obtained in transmission lines with smallersized loading coils.

It is understood the invention is not to be limited to the exactembodiments above described, but only within the scope of the appendedclaims.

What is claimed is:

1. The process of making a low loss magnetic body from finely dividedmagnetic material of the type requiring heat treatment at a hightemperature to properly develop its magnetic characteristics, andinsulating material, which consists in first applying coatings of saidinsulating material to the individual particles of the magneticmaterial, then heat treating the insulated magnetic particles at a hightemperature to anneal the magnetic material therein and to removevolatile constituents from the insulating coatings,

compressing the resulting insulated particles into the desired form, andthen heat treating the compressed body at as high a temperature as theinsulating material will withstand without substantial deteriorationfrom an in sulating standpoint, to remove the strains Inwitness'whereof, I hereunto subscribemy name this 28th day of April,1932.

LOWELL M. HOLLINGSWORTH.

produced in the magnetic material in the forming operation.

2. The process of making a magnetic body from finel divided magneticalloy including nickel an iron, and an insulating and binding material,which consists in first applying coatings of the insulating and bindingmaterial to the individual alloy particles, then heat treating theinsulated alloy particles at a high temperature to anneal the magneticmaterial therein, and to remove volatile constituents from theinsulating coatings, compressing the resultant insulated alloy particlesto produce a substantially homogeneous body in the desired form, andthen heat treating the formed body at the highest temperature theinsulating and binding material will withstand without substantialdeterioration, to remove the strains produced in the magnetic materialby the forming operation.

3. In the process of producing a ma etic body comprising finely dividedpartic es of a magnetic alloy including nickel and iron and insulatingmaterial therebetween compressed into a substantially homogeneous massand subsequently heat treated at a high temperature to anneal themagnetic alloy material,

nealing heat treatment and thus improving istics which consist of themagnetic body its magnetic characterin subjecting the mag-

